Heretofore, sensors such as biosensors and pH sensors with an FET have been proposed (see Patent Literatures 1 to 3). Generally, with a sensor that utilizes an FET, a source electrode and a drain electrode are electrically via a channel formed on an insulating film. A reaction field for a detection target substance is formed on the channel. A reaction membrane for immobilizing a detection target substance is provided on the reaction field. Then, normally, a gate voltage is applied by means of a gate electrode arranged on the reaction membrane, and then the concentration or the like of a detection target substance provided in the reaction field is measured by detecting a source-drain current at that time.